In a therapy using radiation, increased is a demand for a particle beam therapy apparatus using a particle beam (charged particle beam) that is represented by a proton beam or a carbon ion beam having high dose concentration ability on a tumor cell to be a target.
Even in the particle beam therapy apparatus, there is a need to irradiate a tumor region with a specified dose so as to concentrate the dose on the tumor region as precisely as possible. As a method for concentrating the dose in the particle beam therapy, use of a scanning method is widely spread. The scanning method is a method in which an inside of the tumor is irradiated so as to be fully filled by guiding a fine particle beam to an arbitrary position within a plane, and a high dose is assigned only to the tumor region. In a case of the scanning method, there is basically no need for a patient-specific instrument such as a collimator, and there is an advantage to be capable of variously forming distribution.
A radiation therapy planning apparatus is an apparatus that simulates dose distribution in a body of a patient by numerical value calculation, based on information in the body of the patient obtained from a CT image or the like. An operator determines irradiation conditions such as an irradiation direction of the particle beam, beam energy, an irradiation position and an irradiation quantity while referring to a calculation result of the therapy planning apparatus. Hereinafter, a general process thereof will be briefly described. In the scanning irradiation, there are a spot scanning method and a raster method, but here, a case on the assumption of the spot scanning method will be described.
First, the operator inputs a target region to be irradiated with the radiation. If necessary, the operator similarly inputs and registers a position of an important organ where the irradiation quantity of the radiation is suppressed to be low as far as possible.
Next, the operator sets a prescription dose which becomes a dose value to be an aim with respect to each of the registered regions.
Subsequently, the irradiation condition realizing the dose distribution which satisfies the prescription dose is determined. The operator adjusts a parameter relating to the irradiation condition to be determined by using the therapy planning apparatus until the dose distribution which is contemplated to be proper is obtained. In order to efficiently determine the parameter, a method for using an objective function that quantifies a deviation from the prescription dose is widely adopted.
As one of a method for calculating the dose distribution of the proton beam in the therapy planning apparatus, there is a simplified Monte Carlo algorithm (NPL 1). In the simplified Monte Carlo algorithm, since transport calculation is performed with respect to beam particles one by one in the same manner as a normal Monte Carlo algorithm, it is possible to calculate the dose distribution with high accuracy in a nonhomogeneous medium.
In the transport calculation, trajectories of the particles are dividedly connected to minute steps, and in each step, a very small change is given in a track direction depending on multiple Coulomb scattering. A scattering angle of the multiple Coulomb scattering is modeled by a random number of Gaussian distribution, and a standard deviation thereof is calculated per step by using Highland's formula or the like.
Here, in NPL 2, written is a need to contemplate a dose component due to the particles which are scattered at a large angle by a nuclear reaction or the like, in order to secure sufficient calculation accuracy, in the dose calculation of a scanning irradiation method.